© 2012 pearson education, inc. 4 c h a p t e r carbohydrates: plant-derived energy nutrients and in...
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© 2012 Pearson Education, Inc.
4C H A P T E R
Carbohydrates:Carbohydrates:Plant-Derived Energy Plant-Derived Energy
NutrientsNutrientsand In Depthand In Depth
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What Are Carbohydrates?
Carbohydrates One of the three macronutrients An important energy source, especially for nerve
cells Composed of the atoms carbon, hydrogen, oxygen Good sources include fruits and vegetables
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What Are Carbohydrates?
Glucose The most abundant carbohydrate Produced by plants through photosynthesis Found in plants as a component of disaccharides
and complex carbohydrates Principal form of carbohydrate found in blood
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Photosynthesis
Figure 4.1
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What Are Carbohydrates?
Simple carbohydrates contain one or two molecules Monosaccharides contain only one molecule
Glucose, fructose, galactose
Disaccharides contain two molecules Lactose, maltose, sucrose
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Monosaccharides
Figure 4.2
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Disaccharides
Figure 4.3
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What Are Carbohydrates?
Complex carbohydrates Long chains of glucose molecules Also called polysaccharides Starch, glycogen, most fibers
PLAYPLAY Carbohydrates
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Complex Carbohydrates
Figure 4.4
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Complex Carbohydrates
Starch Plants store glucose as polysaccharides in the form
of starch Our cells cannot use complex starch molecules as
they exist in plants So, we digest (break down) starch to glucose Grains, legumes, and tubers are good sources of
dietary starch
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Complex Carbohydrates
Glycogen Animals store glucose as glycogen Stored in our bodies in the liver and muscles Not found in food and therefore not a source of
dietary carbohydrate
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Complex Carbohydrates
Fiber Dietary fiber: the non-digestible part of plants
Grains, rice, seeds, legumes, fruits
Functional fiber: carbohydrate with known health effects, which is extracted from plants and added to foods Cellulose, guar gum, pectin, psyllium
Total fiber = dietary + functional fiber
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Complex Carbohydrates
Dietary fiber is also classified by solubility
Soluble fiber Dissolves in water Easily digested by bacteria in the colon Found in citrus fruits, berries, oats, and beans Reduces risk for cardiovascular disease and type 2
diabetes by lowering blood cholesterol and glucose levels
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Complex Carbohydrates
Insoluble fibers Generally do not dissolve in water Found in whole grains (e.g., wheat, rye, brown
rice), the husk of grains, and many vegetables Promote regular bowel movements, alleviate
constipation, and reduce risk for diverticulosis
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Complex Carbohydrates
Figure 4.6
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Why Do We Need Carbohydrates?
Energy Fuel daily activity Fuel exercise Help preserve (“spare”) protein for other uses Sources high in fiber reduce risk for obesity
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Why Do We Need Carbohydrates?
Energy Each gram of carbohydrate = 4 kcal Red blood cells rely only on glucose for their
energy supply Both carbohydrates and fats supply energy for daily
activities Glucose is especially important for energy during
exercise During intense exercise, carbohydrate will supply
two-thirds or more of the total energy needed
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Why Do We Need Carbohydrates?
Energy Sufficient energy intake from carbohydrates
prevents production of ketones as an alternate energy source
Excessive ketones can result in high blood acidity and ketoacidosis
High blood acidity damages body tissues
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Why Do We Need Carbohydrates?
Fiber May reduce the risk of colon cancer May reduce the risk of heart disease Can enhance weight loss Helps prevents hemorrhoids, constipation, and
diverticulosis
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Why Do We Need Carbohydrates?
Figure 4.7
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Digestion of Carbohydrates
Salivary amylase An enzyme in saliva Begins the process of breaking starch down to
maltose The acidic environment found in the stomach
inactivates this enzyme
PLAYPLAY Carbohydrate Digestion
PLAYPLAY Carbohydrate Absorption
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Digestion of Carbohydrates
Most chemical digestion of carbohydrates occurs in the small intestine
Pancreatic amylase Enzyme produced in the pancreas and secreted into
the small intestine Enzymatically digests starch to maltose
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Digestion of Carbohydrates
Additional enzymes secreted by cells that line the small intestine (mucosal cells) digest disaccharides to monosaccharides
These enzymes include maltase, sucrase, and lactase
Monosaccharides are absorbed into the cells lining the small intestine and then enter the bloodstream
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Digestion of Carbohydrates
Figure 4.8
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Digestion of Carbohydrates
Most monosaccharides are converted to glucose by the liver
Glucose is released into the bloodstream to provide immediate energy
Excess glucose is converted to glycogen and stored in the liver and muscles
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Storage of Glycogen
Figure 4.9
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Digestion of Carbohydrates
Humans do not have the enzymes necessary to digest fiber
Bacteria in the large intestine can break down some fiber
Most fiber remains undigested and is eliminated with feces
PLAYPLAY Diverticulosis and Fiber
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Regulation of Blood Glucose
The level of glucose in the blood is closely regulated within a fairly narrow range
Two hormones, insulin and glucagon, control the level of glucose in the blood
PLAYPLAY Blood Glucose
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Regulation of Blood Glucose: Insulin
Insulin A hormone secreted by the pancreas Transported in our blood throughout the body Helps transport glucose from the blood into cells Stimulates the liver and muscles to take up glucose
and convert it to glycogen
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Regulation of Blood Glucose: Insulin
Figure 4.10a
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Regulation of Blood Glucose: Glucagon
Glucagon Another hormone secreted by the pancreas Stimulates the breakdown of glycogen to glucose to
make glucose available to cells of the body Stimulates gluconeogenesis—the production of
“new” glucose from amino acids
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Regulation of Blood Glucose: Glucagon
Figure 4.10b
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Regulation of Blood Glucose
Glycemic index A measure of a food’s ability to raise blood glucose
levels Foods with a low glycemic index
Cause mild fluctuations in blood glucose level Are better for people with diabetes Are generally higher in fiber May reduce the risk of heart disease, colon cancer, and
prostate cancer
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How Much Carbohydrate?
Recommended Dietary Allowance (RDA) is 130 g/day just to supply the brain with glucose
45–65% of daily calorie intake should be in the form of carbohydrates
Focus on foods high in fiber and low in added sugars
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How Much Carbohydrate?
Most Americans eat too much added sugar Sugars added to foods during processing or
preparation Most common source is soft drinks Typical sources are cookies, candy, fruit drinks Unexpected sources include peanut butter, flavored
rice mixes, canned soups Added sugars are not chemically different from
naturally occurring sugars, but have fewer vitamins
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How Much Carbohydrate?
Problems associated with eating too much sugar: Can cause dental problems and tooth decay No association with childhood hyperactivity
proven; long-term effects not known Associated with increased levels of “bad
cholesterol” Associated with decreased levels of “good
cholesterol” Does not cause diabetes but may contribute to
obesity
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How Much Carbohydrate?
Most Americans eat far too little fiber-rich carbohydrate
The Adequate Intake (AI) of fiber is 14 grams per 1,000 kcal in the diet daily (or, 25 g for women; 38 g for men)
Whole-grain foods (grains, vegetables, fruits, nuts, legumes) are much more healthful sources than foods with added sugar or fiber
How Much Sugar? Cutting Excess From Your Diet
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Alternative Sweeteners
Nutritive sweeteners Contain 4 kcal energy per gram Sucrose, fructose, honey, brown sugar
Sugar alcohols Contain 2–3 kcal energy per gram Have decreased glycemic response and increase
risk for tooth caries
Non-nutritive (alternative) sweeteners Provide little or no energy Developed to sweeten foods without usual risks
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Alternative Sweeteners
No Acceptable Daily Intake (ADI) has been set for saccharin (e.g., “Sweet ‘N Low”), but it has been removed from the list of cancer-causing agents
ADIs have been established for: aspartame (e.g., “Equal”) acesulfame-K (e.g., “Sweet One,” “Sunette”) sucralose (e.g., “Splenda”)
Jump Start Your Diet: Put More Fiber on Your Plate ?
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In Depth: Diabetes
Diabetes Inability to regulate blood glucose levels Three types
Type 1 diabetes Type 2 diabetes Gestational diabetes
Uncontrolled diabetes can cause infections, nerve damage, kidney damage, blindness, seizures, stroke, cardiovascular disease, and can be fatal
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In Depth: Diabetes
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In Depth: Diabetes
Type 1 diabetes Accounts for about 10% of all cases Body does not produce enough insulin Creates high blood sugar (glucose) levels Key warning sign is frequent urination May lead to ketoacidosis, coma, death May be an autoimmune disease Most frequently diagnosed in adolescents Has a genetic link
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In Depth: Diabetes
Type 2 diabetes Develops progressively over time Body cells become insensitive or unresponsive to
insulin Obesity is most common trigger Variations include insulin resistance, impaired
fasting glucose, and pre-diabetes Eventually the pancreas may become unable to
produce any insulin
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In Depth: Diabetes
Who is at risk? Obesity, genetics, physical inactivity, and poor diet
increase overall risk Metabolic syndrome (high waist circumference,
high blood pressure, high blood lipids and glucose) increases risk for type 2 diabetes
Increased age increases risks, but younger people and even children are now commonly diagnosed
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In Depth: Diabetes
Prevention and control Eat a healthful diet, get daily exercise, keep a
healthful body weight Eating slightly fewer carbohydrates and slightly
more protein and fat may help regulate blood glucose levels; consult a dietician
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In Depth: Diabetes
Prevention and control Avoid alcoholic beverages, which can cause
hypoglycemia Healthful lifestyle choices can prevent or delay
onset of type 2 diabetes Oral medications and/or insulin injections may be
required once diabetes has been diagnosed